First, X11 is a Windowing system for unix based systems, sort of like Aqua on MacOS X but it runs on multiple platforms, OSes, and it is open source ( Apple supports an X11 environment to use on MacOS X, http://www.apple.com/macosx/x11/ ). x86 is a processor architecture, x86 is what most intel (pentium) and AMD processors are. Macs use PowerPC processors.

Redhat Linux is for x86 based processors and wont work on a PowerPC processor. There are many Linuxes similar to Redhat Linux for PowerPC processors, Yellow Dog Linux for example (www.yellowdoglinux.com)

Yes, with Apple's X11 you can get most open source software to work on Mac OS X. Trying to run Linux on the same box will just lead to confusion. Mac OS X has all the Unix goodness, plus a nicer GUI and commercial software (including good games).

Get the X11 beta from Apple's website and install it. Then you can use OpenOffice, FreeCiv, Gimp, etc.

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